Plutonium-238, not to be confused with its weapons-grade variant, Pu-239, powers spacecraft by producing heat through radioactive decay. The method has powered previous missions such as the Viking missions on Mars, the Voyager spacecraft and, more recently, the Curiosity Mars Rover and New Horizons spacecraft.

"Radioisotope power systems are a key tool to power the next generation of planetary orbiters, landers and rovers in our quest to unravel the mysteries of the universe," he said.

Right now, NASA only has access to 35 kilograms, about 77 pounds, of Pu-238 to power space exploration missions. That's just enough to last into the middle 2020s, powering just two or three proposed missions.

So regaining the capability to make Pu-238 has been much needed.

"As we seek to expand our knowledge of the universe, the Department of Energy will help ensure that our spacecraft have the power supply necessary to go farther than ever before," Franklin Orr, Under Secretary for Science and Energy at DOE, said in the government news release. "We’re proud to work with NASA in this endeavor, and we look forward to our continued partnership."

Two years ago, NASA began funding efforts to make Pu-238 again in ernest. The agency has put about $15 million each year toward the DOE's Office of Nuclear Energy's efforts.

The new capabilities aren't yet ready for full production. Researchers still have to verify that they can produce Pu-238 on a larger scale without losing its purity or other characteristics. They may have to make some adjustments to the process.

"Once we automate and scale up the process, the nation will have a long-range capability to produce radioisotope power systems such as those used by NASA for deep space exploration," Bob Wham, who leads the project for the ORNL's Nuclear Security and Isotope Technology Division, said in an ORNL press release.

But, he said, "With this initial production of plutonium-238 oxide, we have demonstrated that our process works and we are ready to move on to the next phase of the mission."